Existing processes used for treating wood with preservatives include the Bethell, Lowry, Reuping and MSU processes.
The Bethell process involves using an initial vacuum to remove air from the wood cells and then flooding with preservative solution a cylinder loaded with the wood under vacuum. Positive pressure of about 1400 kPa is then applied for a predetermined time, the preservative solution is drained and a final vacuum is drawn. All pressures referred to herein are gauge.
In the Lowry process, no initial vacuum is applied and the cylinder is flooded under atmospheric pressure. Positive pressure of about 1400 kPa is then applied for a predetermined period, the cylinder is then drained and a final vacuum drawn. The preservative net uptake is lower because the air is not removed from the wood cells but is compressed during treatment, thus resulting in kickback of preservative when pressure is released and the timber evacuated.
The Reuping process involves applying an initial air pressure of about 350 kPa to the wood in the cylinder and then flooding the cylinder holding this initial air pressure. Increased pressure of about 1000 kPa is then applied and, after a predetermined time, the pressure is released and the cylinder drained. A final vacuum is then drawn. This process has a lower net uptake than both the Bethell and Lowry processes.
The MSU process is a modification of the Reuping process. The Reuping process is carried out but the cylinder is drained maintaining a pressure of about 300 kPa. Heat is then applied by steaming the wood to fix the preservative. After the fixation period, kickback is allowed to occur by reducing the pressure and a final vacuum is drawn.
Pulsation or processes which cycle pressure have also been used to improve the treatment of relatively impermeable wood. Specialised treatment schedules have been developed involving oscillating, alternating or pulsation pressures to improve penetration and hence treatment of impermeable wood. Some of these processes involve higher pressures than is used in the aforementioned conventional treatment plants.
These processes involve rapid changes in pressure and it is believed that this causes a greater pressure difference through obstacles within the wood, while the total pressure within the wood increases slowly allowing the preservative to enter small pores. Care must be taken using very high pressure treatments as the wood cells are likely to collapse.
The oscillating pressure method (hereinafter referred to as "OPM") is suitable for treating wood species such as spruce which are difficult to treat once dry. The process is carried out with an oscillating change of pressure between vacuum and pressure. The pressure range is -93 kPa to 600-1500 kPa. During the pressure phases of the process, preservative solution is forced into the wood where it mixes with the wood sap. During the vacuum cycles, air entrapped in the wood expands, forcing a mixture of wood-sap preservative and air out of the wood. As the cycles continue their is a gradual replacement of wood sap in the wood with preservative solution.
The wood to be treated by the OPM must usually be sap fresh (green), meaning the moisture content must be above fibre saturation in all parts of the sapwood. Air must be present to expand during the vacuum phase and escape from the wood so that the sap can be sucked out of the wood and the impregnating solution pressed into it.
The OPM can be carried out on easy to treat species, such as pine in semi-dry or fully dry condition. The time to treat air dry poles by the OPM is two to four hours compared to 14 to 18 hours for sapfresh pine poles. For dry wood, the OPM gives approximately the same results as the Bethell process. Considerably improved impregnation is obtained on unseasoned wood.
In New Zealand, the OPM has been successfully used to treat pine species after steam conditioning. It had been found that freshly cut pine was too saturated to be treated green by the OPM.
The OPM process was modified in New Zealand to exclude the vacuum phase. The resultant process, known as the Alternating Pressure Method (hereinafter referred to as "APM"), involves a number of cycles at pressure from 0-1400 kPa. This is equivalent to a series of Lowry empty cell treatments.
The APM is possible because of the action of steam preconditioning. Species used in New Zealand with the APM are P. radiata and P. nigra.
Initial APM schedules required one hour cycling for about every 2.5 cm of sapwood depth. Later research showed that 15 cycles were sufficient for complete sapwood penetration. The heartwood of sawn timber is treated partially by cycling and further by maintaining the final cycle on pressure for an extended time. The cylinder is flooded without an initial vacuum and then the APM cycles are 1 to 2 minutes on pressure at 1400 kPa and 1 minute off pressure.
The pulsation process is a further modification of the OPM. It was developed to increase the penetration in refractory species like white spruce. Pulsation trials using both creosote and water-borne CCA (copper-chrome-arsenic preservative) have been conducted with white spruce roundwood and sawn timber.
The pulsation process alternates between high and low pressures of 300 kPa to 2100 kPa. 2100 kPa is well above the normal pressures used for treating wood. The aim of pulsation is to treat refractory species. These species may also be prone to collapse.
Pulsation is based on the Reuping process with a sequence generally as follows:
(a) Initial air pressure of 350 kPa. PA1 (b) Cycling between 350 kPa and 2100 kPa. Some of the schedules involve increasing the pressure to 2100 kPa over several cycles i.e. first to 1000 kPa, second to 1200 etc. up to 2100. This slow rise is to minimise collapse caused by the high pressure. PA1 (c) The cylinder is then drained and a final vacuum drawn. PA1 introducing wood to be treated into a treatment vessel; PA1 optionally applying an initial vacuum or pressure to the wood, the pressure if applied being less than 150 kPa; PA1 immersing the wood in the vessel in a waterborne preservative and treating the wood by impregnating the wood with the preservative, the treatment being conducted at elevated temperature of from greater than 30.degree. C. to less than 100.degree. C. so as to facilitate fixation of the preservative in the wood, and at elevated pressure in the treatment vessel to facilitate impregnation of the wood by the preservative; PA1 separating the impregnated wood and the excess waterborne preservative while the treatment vessel is pressurized; and PA1 reducing the pressure in the treatment vessel. PA1 (a) there is more wood in a charge as fillets take up space; PA1 (b) it is less costly than filleting and destacking; and PA1 (c) it is easier to handle the package. PA1 impregnating the wood with a waterborne preservative using a modified Bethell process; and PA1 subsequently impregnating the wood with oil, said oil impregnating step being performed under pressure and said oil being heated.
Total treatment time varies between 7 and 20 hours depending on the number and duration of the cycles. Improvement in the treatment of refractory spruce has been achieved.
The Fast process was developed in New Zealand to increase productivity in treatment plants. The process involves the use of 5 cycles of pressure from 0 to 1400 kPa, i.e. a short APM. However, instead of using steam preconditioned timber, air dried or kiln dried timber is treated.
Less time is taken in treating the timber because there is no initial vacuum. The process was validated by carrying out trials with matched samples treated by a Bethell process. It was found that there was no significant difference in penetration or retention between the Fast and Bethell processes. The fast process is now used by a number of plants in New Zealand.
The aforementioned existing processes for the fixation of waterborne preservative such as CCA to wood involve two distinct steps. The first step involves treatment of the wood with the preservative at about ambient temperature and then removal of the treatment solution.
The second step involves fixation by heating the treated wood at moderate or high temperatures or at low temperatures for a long period of time. For example, in the MSU Process the treated wood is subjected to hot water and steam at about 95.degree. C. to accelerate fixation of the preservatives.
A problem with the aforementioned existing processes is that the two step operation necessitates the use of a complex plant operation, the treatment and fixation time is prolonged, and there is a risk that not all of the preservative is fixed to the wood which can cause leaching of harmful preservatives to the environment.
There is considerable pressure on the wood preservation industry to ensure that all treatment plants meet environmental standards. In most cases this will mean the introduction of a fixation step which may be as simple as drip pads to hold the timber at ambient temperature until fixation or a separate fixation process.
One potential method of reducing the fixation time of waterborne preservative to wood is to treat the wood with the preservative at elevated temperature. For example, A. Pizzi in "A New Approach to the Formulation and Application of CCA Preservatives", Wood Sci. Technol. 17 (1983) at 304-307 confirms that an increase in treating temperature increases the rate of fixation. However, treatment of wood with waterborne preservatives at above-ambient temperatures has not been practised except in countries with very cold winters when the solution may be warmed to about 20.degree. C. In some cases, such as CCA, this is because it has long been believed that the waterborne preservative is unstable at elevated temperature.
We have found that CCA is in fact stable at elevated temperatures unless the solution is contaminated with a reactant which converts the hexavalent chrome to trivalent chrome and causes precipitation and consequent sludging of the solution. Such reactants include the wood sugars which appear in kickback from the treated wood when pressure is removed.
We have now found in a first aspect of the invention that applying heated waterborne preservatives such as CCA to wood can achieve rapid fixation which alleviates the cost and environmental problems associated with the existing processes and that kickback contamination can be alleviated.
Creosote is a heavy oil of tar which has been widely used as a wood preservative which imparts water repellency and dimensional stability to wood. Creosote contains a vast array of organic chemicals some of which are very toxic. The environmental risks involved in using creosote are now being recognised. Furthermore, creosote is costly and difficult to manage on a commercial scale.
The treatment of wood with zinc chloride and creosote is known. However, this two stage treatment lost favour due to the high costs involved in using creosote.
The use of two stage treatments where the wood is allowed to dry between treatment with a waterborne preservative and creosote have also been investigated. However, such treatments were found to be too costly and time consuming.
Water repellent copper-chrome-arsenic (hereinafter referred to as "CCA") emulsions have also been produced. This has been achieved by the addition of water repellents, such as, waxes and resins to the CCA. Emulsions of CCA and oil have also been developed. Both the CCA/water repellent and CCA/oil emulsions have limitations due to the high costs involved in producing them and the need to store them in special tanks. In some instances, the emulsions have also been found to break down.
A requirement accordingly exists in a second aspect of the invention for a wood preservative process which enhances the water repellency of the wood, but which avoids or at least alleviates the environmental and cost problems described above.